Former high-ranking employees from the company that manages Australia's immigration detention centres say systemic mismanagement of taxpayer funds is being "swept under the table" by the Government.

In a series of explosive allegations, two former employees of the private security firm which has a $756 million contract with the Federal Government claim fines for contract breaches at detention centres are being pushed to the side "for political reasons".

The employees have also detailed lax security practices at the Christmas Island detention centre.

The Government fines the security firm, Serco, for any breach of its contract, which can include detainee escapes, riots, or untimely transport escorts.

But despite an extensive audit system, the Christmas Island insiders claim the financial penalties, or abatements, are not always recouped by the Department of Immigration and Citizenship (DIAC).

"The monthly abatements ran into the many hundreds of thousands of dollars [on Christmas Island] but when it would reach up into the cluster management or regional manager level, they would sort it out with DIAC and nothing would ever come to pass,'' a former Serco manager told ABC's Online Investigative Unit.

"The view among senior management on the ground, who were worried about it, was that they were sweeping it under the table, probably for political reasons, as they probably didn't want it getting around how bad the situation was there.

"When it got really bad, the amount mentioned that Serco were going to be abated [on Christmas Island] was $1.5 million, and always at the end of the month after they had the final abatement meeting, but it would just be pushed off to the side.

"I'm sure there's a lot at stake to make it look like Serco is coping or just coping but it is just wrong for Australian taxpayers that these people are gilding the lily."

In response to the allegations, a DIAC spokesperson said the abatements could not fall through the cracks.

"The Department of Immigration and Citizenship follows up all breaches at all immigration detention facilities and these are taken very seriously."

DIAC serves abatements against Serco once a month for unfulfilled contractual obligations yet these are "commercial in confidence" and not publicly disclosed.

Serco did not return calls to the ABC prior to publication.

Waste of money

One former Serco employee alleges a range of contractual obligations were not being met on Christmas Island but that DIAC was unaware.

"The wastage of money and lack of accountability was concerning.[Serco] staff could put down extra hours and they wouldn't even know where staff were - people claiming wages and they weren't even on the island," the former employee said.

The allegations coincide with calls by the Opposition for a parliamentary inquiry into Australia's immigration detention system.

The call is backed by the Greens and by independent MP Andrew Wilkie.

The Greens also want a broader inquiry into mandatory detention and requested that DIAC table Serco abatements in last week's Senate estimates hearing.

"You wouldn't have to be too clever to find a whole host of financial and human resource mismanagement, it would be plain to see," the former Serco manager said.

"They would just have to ask for the records of the contract management meetings. They are all minuted and recorded, pages and pages of evidence."

It is understood that acts of non-compliance such as the escapes, riots and fires at Christmas Island in mid-March incur some of the highest penalties.

"There were the times when there were the big escapes and the damages occurring," the whistleblower continued.

"Certainly the figures raised at meetings that I attended, there was the potential to be abated well over $1 million. That's for one month."

The other insider added: "You could be greeted by a security officer sitting and having a cigarette and that's when you walked in the gate. And I'd say, 'Well, aren't you going to search my bag?' It was just not up to scratch.

"You were not supposed to take cameras into the property. I had a camera in my bag since day one. People are supposed to X-ray your bag coming and going. No. Not always.

"There were a whole heap of people wandering outside the camps almost on a daily basis, at will. What they were saying is that these people had escaped because the fences weren't secure.

"But what was occurring and it was common knowledge among the people there - is that there were people [detainees] who were just wandering in and out of the camps."

In last week's Senate estimate hearings, Greens Senator Sarah Hanson-Young drew attention to the lack of transparency of the contract, breaches and abatements.

DIAC official Fiona Lynch-Magor responded: "The Serco contract provides significant capability for the Department to ensure that the contract is appropriately administered."

"We certainly have abated Serco [in] the period of this contract on many occasions for their failure to deliver the contract but it is not recorded in a recordable manner, as in X number of breaches this financial year, because of the way the abatements work."

The Serco whistleblower says a paper trail would not be hard to locate.

"All somebody needs to say to DIAC is, we would like to see a copy of their minutes from the abatement meetings that are held every Thursday at approximately 2:00pm [on Christmas Island]."